It is currently not economical to remove hydrogen from low pressure (&lt;125 psig), low concentration streams present in oil refinery applications. The currently practiced process for using such streams is to blend them with natural gas to burn and recover the heating value from the hydrogen present. There are several current alternative methods for hydrogen generation in such a situation. Steam methane reformer generation of hydrogen has high capital and energy costs relative to the proposed process based on economic evaluation at 25 MMSCFD hydrogen production. Thus, steam methane reformer hydrogen generation applicability is limited to very large systems. Adsorption based purification of these lower pressure hydrogen streams with current technology is not effective since the resulting recoveries and product purities are extremely low. Recent adsorption process patents report only 76% recovery of only a 75% pure hydrogen product, starting with a 52% hydrogen feed.
Semipermeable membrane based purification of comparable purity low pressure feed streams would have prohibitively high compression costs, since so much of the feed is waste gas. Also, membrane based systems have very high capital costs associated with medium-to-large scale operation so they would not be suitable for the applications addressed by the current adsorption based process. Thus, there is a need for a low cost process for hydrogen recovery from low grade waste streams to meet the medium-to-large hydrogen demands in oil refineries and other similar environments.
Recent changes in environmental regulations in association with the Clean Air Act have forced liquid fuel producers to acquire more hydrogen to process crude oil into more clean burning fuel formulations. This increased hydrogen demand must be met by new sources in oil refinery and similar environments so a greater need for new methods of hydrogen purification of production now exists.
U.K. Patent 2,154,465 discloses a vacuum swing adsorption process for recovering hydrogen in waste gas obtained in petrochemical plants. The vacuum swing adsorption process described is conducted through a series of stages, including adsorption, simultaneous cocurrent depressurization to provide pressure equalization gas while countercurrently venting, providing purge to an evacuated bed, evacuation, purge under vacuum, product repressurization, and pressure equalization. The patented process has discontinuous feed, while the dual end step provides equalization gas and rejects waste. The patent also uses product repressurization before a final bed-to-bed equalization. The result of this process is that with a feed of 52% hydrogen, 42% nitrogen, 5% carbon monoxide and 1% carbon dioxide, the resulting hydrogen product is only 75% pure at a recovery of only 76%.
U.K. Patent 2,155,805 discloses a vacuum swing adsorption process for recovering hydrogen from flue gases from petrochemical plants, reduction furnaces and annealing furnaces. This patent uses a similar cycle sequence to the above U.K. patent, but includes a feed gas and product gas repressurization step. The patent describes a cocurrent depressurization to provide pressure equalization gas while simultaneously countercurrently venting the appropriate adsorption bed.
U.S. Pat. No. 4,981,499 describes a hydrogen pressure swing adsorption process in which cocurrent depressurization of the bed is made to a separate reservoir which supplies purge gas for other beds. The cocurrent depressurization is conducted while performing a countercurrent vent of the former bed. However, purging of a bed supplied with gas from the separate reservoir is not conducted at the same time as the cocurrent depressurization and countercurrent vent.
Additional references of interest in the field of pressure swing adsorption include U.S. Pat. No. 4,402,712; U.S. Pat. No. 4,969,935; U.S. Pat. No. 4,589,888; U.S. Pat. No. 4,650,501; U.S. Pat. No. 4,684,377; U.S. Pat. No. 4,917,710 and U.K. 2,237,220.
The shortcomings of the prior art are overcome by the present invention which provides a unique solution to the problem of high recovery and high purity hydrogen from low pressure, low purity hydrogen-containing gas streams, such as refinery off gases, as will be set forth in greater detail below.